This invention relates to touch sensitive panels used in electronic devices, such as computers, hand held PDAs and radiotelephones.
Touch sensitive panels, referred to as xe2x80x9cinput screensxe2x80x9d herein, are widely used for hand held devices, including personal digital assistants (PDAs), radiotelephones and other hand held wireless devices. An input screen with a resistive overlay (e.g., indium tin oxide) includes upper and lower transparent input layers positioned above a display screen, where each input layer includes two electrodes and a sequence of parallel electrically resistive paths between the two electrodes. When a user uses a stylus or other appendage to touch the input screen at a selected location, the upper and lower input layers contact each other, generating a signal that identifies an x-coordinate (horizontal) and a y-coordinate (vertical) for the contact point (touched location) relative to the display screen image. A conventional input screen requires at least four wires, one for each electrode, to transfer location information signals from electrode to a signal processor that analyzes these signals. A conventional input screen will group all four wires as a unit and will route the wires along several edges of the input screen to the appropriate electrode. The device usually has one relatively inflexible tail that includes the four wires. This tail is usually bulky and requires provision of additional room around the input screen in which to fit the tail.
One result of this approach is that presence of the tail requires provision of a relatively large, non-usable, referred to herein as a xe2x80x9crouting zonexe2x80x9d, on one or two sides of the four sides of the input screen, to provide room for the tail. A second result of this approach is that the portion of the device housing that surrounds the input screen is non-symmetric, being noticeably wider on one or two sides than on the opposite side(s). A third result is that the key area or active area (bounded by the four electrodes; the region where the alphanumeric characters and graphics appear on the display screen) is reduced substantially, often by as much as 10-14 percent, relative to the input screen key area that would be available if the four-wire tail were not present. A fourth result of this approach is that the tail, when received within the device housing, is relatively inflexible and cannot be easily reconfigured to fit into the routing zone for wiring of the input screen and other components.
Many of the input screen systems rely on alternatingly switching off (isolating) and switching on one of the two pairs of opposed electrodes, in order to estimate the currents present at the input screen contact point. An example of this approach is disclosed in U.S. Pat. No. 4,293,734, issued to Pepper and incorporated by reference herein. Provision of electrode switching requires more complexity in the signal processor and associated hardware used to estimate the (x,y) coordinates of the input screen contact point. Electrode switching also requires that the electronics system be allowed to re-settle (in a time estimated to be xcexcsec to msec) before another current measurement may be taken.
What is needed is an approach that (1) does not require switching of electrodes in order to determine the (x,y) coordinates of the input screen contact point, (2) does not require provision of a settling time before one or more current measurements can be made, (3) has a simpler construction than man of the prior art input screen systems, (4) allows use of arbitrary resistance values for the resistive lines used on each of the upper and lower input layers, (5) allows use of two wires, three wires or four wires connecting the electrodes and upper and lower input layers to a signal processor used to determine the contact point coordinates, (6) allows an increase in the size of the input screen key area or active area, (7) allows a reduction in one or more dimensions of a hand held computing device that employs this input screen system, and (8) allows the key area to appear in a symmetric and more pleasing arrangement as part of the device housing.
These needs are met by the invention, which provides a method and system for providing a input screen in which a current and a voltage, or two currents, are measured adjacent to electrodes or other electrical contact points having substantially constant (unswitched) voltages. Resistance values associated with two resistive line segments that extend from an input screen contact point to the four electrodes are then determined, and (x,y) coordinates for the contact point are determined. Electrode voltage switching is not performed, and the total number of wires required to be connected to the input layers is three, including two wires that provide voltages for the electrodes. The total resistance value for resistive lines extending from one electrode to an opposed electrode is arbitrarily chosen.
One benefit of this invention is simplicity: no voltage switching is required, and hence no voltage and current settling times need be provided. Another simplification is use of three wires: the routing surrounding the input screen key area is smaller, the size of the input screen key area can be made larger, one or more dimensions (length and/or width) of the device housing for a hand held computing device with input screen can be reduced without increasing another dimension, and the key area may have a symmetric and more pleasing arrangement relative to the remainder of the device housing. The software and/or firmware used by the signal processor to determine the (x,y) coordinates of the input screen contact point is somewhat more complex, but the computations need only be done once for each new contact point chosen.
In a first embodiment, the resistive lines in each of the first and second input layers are connected at a first end to a selected voltage source and are allowed to xe2x80x9cfloatxe2x80x9d at a second end. The resistive lines for the second input layer are connected across a selected resistor to the second voltage source, which may be ground or some other selected voltage value. This embodiment requires only two electrodes, not four. A current is measured from the first input layer electrode to an electrical contact point, where a resistive line from each of the first and second input layers contact each other, as a result of touching the screen with a stylus, finger or other appendage. Voltage at the contact point is also measured. From these two measurements, the resistance value for each of the two resistive line segments (horizontal and vertical) that connect the contact point to the voltage source for that input layer are calculated, and the corresponding (x,y) coordinates for the contact point are determined.
In a second embodiment, the resistive lines in each input layer are connected between a first voltage source and a second (lower) voltage source. The contact point voltage and a current in one of the four resistive line segments (between a voltage source and the contact point) are measured. From these two measurements, the resistance value for each of two resistive line segments (horizontal and vertical) that connect the contact point to a voltage source are calculated, and the corresponding (x,y) coordinates for the contact point are determined. Each of these two embodiments requires a current measurement, a voltage measurement and a wire for each of two voltage sources.